Americans are facing what may be the largest outbreak of West Nile virus since the disease was first detected in the U.S. in 1999, the CDC said.

With 1,118 cases and 41 deaths reported so far, Americans are facing what may be the largest outbreak of West Nile virus since the disease was first detected in the U.S. in 1999, the CDC said.

As of Aug. 21, 2012, "there have been more cases reported to us than ever before," said Lyle Petersen, MD, director of the agency's Division of Vector-Borne Infectious Diseases.

If the trajectory continues, "this will be amongst the biggest or the biggest outbreak we've ever experienced in the United States," Petersen told reporters in a telephone briefing.

The pace of the disease has picked up markedly, Petersen said, from a mere 25 human cases a month ago.

"Cases now being reported reflect infections from a week or more ago," Petersen said. "We expect many more cases to occur and the risk of West Nile virus infection will probably continue through the end of September."

All told, he said, 47 states, with the exception of Alaska, Hawaii, and Vermont, have reported West Nile activity in people, birds, or mosquitoes, while 38 states have reported human cases.

Five of those 38 states -- Texas, Mississippi, Louisiana, South Dakota, and Oklahoma -- account for 75% of the reported cases and Texas alone accounts for almost half.

"Texas has really been the center of this outbreak," with 526 cases and 21 deaths, said David Lakey, MD, commissioner of the Texas Department of State Health Services.

The hardest-hit region of the state is Dallas County, Lakey said, with 270 cases and 11 deaths. That latter number is "more deaths than ever before," outstripping the total of 10 seen in Dallas County from 2003 through 2011, he said.

Petersen said it is hard to predict the magnitude of an outbreak "until it's almost over," and warned Americans to continue to take precautions against mosquitoes.

Those precautions include:

Using insect repellent

Wearing long pants and shirtsleeves at dawn and dusk

Repairing screens on windows and doors

Emptying pools of standing water

The numbers reported to the CDC may be an underestimate, Petersen said, because cases of West Nile fever may not be reported to doctors and, if they are reported, may be misdiagnosed by the physician.

But reported numbers of neuro-invasive West Nile disease, such as meningitis or encephalitis, are usually pretty accurate, he said, simply because most require hospital admission. "The reporting is very, very complete," he added.

So far this year, 56% of the reported cases (629) have been neuro-invasive, Petersen said.

In Texas, Lakey said, 323 of the reported cases have been neuro-invasive, including 142 of the 270 cases in Dallas County. By comparison, Lakey said, the state's worst previous season was 2003, which ended with a total of 439 neuro-invasive cases and 40 deaths.

Lakey said the state, using a combination of federal and state funds, is using two airplanes to spray for mosquitoes in Dallas County, replacing land-based spraying. Other areas of the state may also request aerial spraying, he said.

Usually in West Nile outbreaks, Southern states bear the brunt, Petersen said, and "we're following that pattern."

The factors behind the outbreak probably include an early spring, a hot summer, and a mild winter, Petersen said, but it's difficult to pin down exact causes. "We don't really know why it's worse this year than in previous years," he said.